The rate of heat transfer through a gas is a function of the gas pressure. Thus, under certain conditions, measurements of heat transfer rates from a heated sensing element can, with appropriate calibration, be used to determine the gas pressure. This principal is used in the well-known Pirani gauge.
Many Pirani gauges comprise temperature sensitive sensing and compensating resistances in separate legs of a Wheatstone bridge. The compensating resistance is sized to minimize self-heating with current applied through the two resistances. The resultant resistance differences with heating of the sensing resistor is indicative of pressure of the surrounding environment.
In more recent heat loss gauge implementations presented in U.S. Pat. No. 6,658,941, the sensing element and compensating element are of like dimensions, but an additional heating current is applied to the sensing element to boost its temperature. Again, the relative resistances of the sensing and compensating elements with increase in temperature of the sensing element are indicative of the pressure of the surrounding environment. One implementation relies on a Wheatstone bridge, while another relies on a fixed ratio of current flow through the resistive elements under control of a feedback circuit responsive to the sensed resistances.